This invention relates generally to semiconductor devices, and more specifically to a process for the formation of electrical isolation structures in a semiconductor integrated circuit.
The trend in semiconductor device fabrication is toward increasing density of circuit components. As the function being implemented in the integrated circuit becomes more complex, an increasing number of devices including transistors, resistors, and the like, are required, and these devices must, of course, be readily manufacturable and reliable. For a given chip size, an increase in the number of active circuit components requires that they be placed in close proximity to each other, thus forcing a corresponding reduction in the surface area of the circuit that can be occupied by electrical isolation structures. The demand to reduce the chip surface area consumed by electrical isolation structures, while maintaining the necessary electrical isolation of adjacent active components, has lead to the development of several different isolation schemes. The most common isolation fabrication technique is a process known as localized oxidation of silicon (LOCOS). This method offers high reliability and proven high volume manufacturing compatibility. A major problem with this technique, however, is the loss of active surface area caused by a lack of ability to control the process. A phenomenon known as lateral encroachment occurs, wherein the final width of isolation oxide is larger than the intended width defined by a patterned silicon nitride mask. The problem of lateral encroachment is illustrated in FIG. 1.
Shown in FIG. 1, in cross section, is a portion of a semiconductor substrate 10 which has already undergone some of the processing steps used to fabricate an isolation structure using a standard LOCOS process in accordance with the prior art. A thick isolation oxide layer 12 has been formed in exposed regions of substrate 10. The oxidation has not occurred in regions of substrate 10 covered by a previously patterned composite layer of silicon nitride 14 overlying a thin layer of pad oxide 16. The lateral encroachment, commonly known as a bird's beak, is denoted in FIG. 1 by the distance labeled X. The degree of encroachment is often related to isolation oxide thickness near the edge of the nitride oxidation mask, denoted as H in FIG. 1, and the total oxide thickness denoted as Tox. The oxidation under the nitride mask layer occurs when oxygen diffuses through the pad oxide layer 16 and reacts with the silicon underlying nitride layer 14.
The desired to suppress the lateral oxidation has lead to the development of process techniques which are intended to retard the oxygen diffusion through the pad oxide layer. One approach is to reduce the thickness of the pad oxide layer to some minimal value, thus reducing the cross sectional area available for oxygen diffusion. Prior to nitride deposition a layer of polysilicon is deposited over the thin pad oxide layer. The layer of polysilicon is used to relieve compressive stress in the substrate. This method, known as poly buffered LOCOS or PBL, achieves a slight reduction of oxide encroachment, however the process is difficult to control and requires removal of both nitride and polysilicon following the oxidation process.
Other techniques have been developed wherein the edge of the pad oxide is covered with a second layer of nitride, or an oxide layer, to protect the pad oxide from exposure to oxygen during the oxidation process, and to provide mechanical resistance to the oxidation of the silicon underlying the nitride layer. Processes known under the acronyms of SILO, SWAMI, and framed mask, are representative examples. These techniques require multiple film depositions and etch back steps making the processes complicated and difficult to perform with high reliability. Additionally, stress related failures associated with the rigorous oxidation process used to form isolation oxide layers are commonly observed. For example, deformed isolation structures are often observed as a result of ruptures of the seam between the masking nitride and the nitride or oxide cap. Attempts to improve upon these techniques and reduce stress related failures, such as rupturing of the sealing layer along the seam, have lead to the development of encapsulation techniques, for example, as described by G. Pollack in U.S. Pat. No. 4,580,330. Using this method, a cavity is formed under the edge of the nitride oxidation mask, between the nitride and the substrate, by etching back the pad oxide layer. This cavity is then filled by the a composite layer formed by a subsequent oxidation of the substrate and a second nitride deposition to prevent the oxidation of the silicon substrate near the edge of the nitride oxidation mask. This complex method requires great care in the selection of oxide and nitride layer thicknesses and involves difficult etching processing steps. Furthermore, because silicon nitride is used as an encapsulation material, this method does not solve the problem of stress relief. Accordingly, a need existed for a defect-free LOCOS process that effectively reduces the lateral oxidation encroachment in a minimum number of process steps having high reliability and manufacturing integrity.